In a variety of environments, seismic surveys are performed to gain a better understanding of subterranean geological formations. In marine environments, for example, seismic surveys are conducted to improve the understanding of geological formations located beneath a body of water. In seismic survey applications, airguns are sometimes employed as seismic sources. The airguns are impulsive sources which use a controlled volume of high pressure air suddenly vented into the water. In other applications, marine seismic vibrators serve as continuous seismic sources that generate a peak output pressure substantially lower than airguns and thus the vibrators are expected to have a lower impact on marine mammals and other environmental considerations. To generate energy comparable to that generated by airguns, the marine vibrators generally emit their signals (sweep) for a long period of time; or several marine vibrators are simultaneously activated.
In towed marine applications using seismic vibrators, the maximum sweep length is equal to the shot point interval divided by the vessel speed. However, there are operational and economic limits to reduction in vessel speed. In marine acquisition using the same vessel to tow the streamer and the marine vibrator (the most common acquisition scenario), the operational limit is due to the need for a minimum vessel speed to keep the streamer afloat. The economic limit is due to the fact that a reduction in vessel speed increases the acquisition time and thus the acquisition cost.